Fuel cell and method for producing same

ABSTRACT

An aim of the invention is to provide a stable fuel cell having a high mechanical strength and a high reliability. Another aim of the invention is to provide a fuel cell which can be easily produced.  
     The fuel cell comprises a porous electrically-conductive material ( 13 ) as a substrate, a protonically-conductive membrane ( 16 ) formed on the porous electrically-conductive material ( 13 ) made of a mesoporous thin film comprising as a main component a crosslinked structure having a metal-oxygen skeleton having an acid group connected to at least a part thereof and having pores periodically aligned therein and a porous electrically-conductive material layer ( 17 ) formed on the protonically-conductive membrane.

TECHNICAL FIELD

The present invention relates to a fuel cell and a method for producingsame.

BACKGROUND ART

In recent years, fuel cells have been noted as a next-generationelectricity-generating apparatus which can make contributions to thesolution to environmental problems and energy problems that are sociallygreat assignments because they exhibit a high electricity- generatingefficiency and excellent environmental characteristics.

Fuel cells are normally classified into several types by the kind ofelectrolyte. Among these types of fuel cells, direct methanol fuel cell(hereinafter referred to as “DMFC”) which directly receives methanol asa liquid fuel to cause electrochemical reaction by which it can workwithout using any reformer.

DMFC can use a liquid fuel having a high energy density and thusrequires no reformer. Therefore, DMFC provides a compact system.Accordingly, DMFC has been noted particularly as an electric supply forportable apparatus substituting for lithium ion battery.

In DMFC, the following electrochemical reaction occurs to cause methanolto react directly with water on the anode and produce water on thecathode.

Anode: CH₃OH+H₂O→CO₂+6H⁺+6e⁻

Cathode: 6H⁺+2/3O₂+6e⁻→3H₂O

Herein, the protonically-conductive membrane acts to transmit protonproduced on the anode to the cathode. The movement of proton occurs incompetition with the flow of electron. In order to obtain a high output,i.e., high current density, it is necessary that the protonic conductionbe conducted in a sufficient amount at a high rate. Accordingly, theperformance of the protonically-conductive membrane greatly governs theperformance of DMFC. Further, the protonically-conductive membrane notonly plays a role in the conduction of proton but also acts as aninsulating film for electrically insulating anode and cathode from eachother as well as a fuel barrier for preventing the fuel supplied intothe anode from leaking to the cathode.

As a high performance protonically-conductive membrane there has beenheretofore used a fluororesin such as perfluorocarbonsulfonic acidpolymer (Nafion®) (Patent Reference 1).

In such a protonically-conductive membrane, some sulfonic acid groupsare agglomerated to form a reverse micelle structure. Therefore, such aprotonically- conductive membrane is disadvantageous in that it caneasily swell to cause methanol crossover as diagrammatically shown inthe structure of perfluorocarbonsulfonic acid polymer before and duringoperation in FIG. 8. In other words, a protonically-conductive channel103 is formed in a reverse micelle structure formed by a sulfonic acidgroup 102 connected to a perfluoro chain 101.

As can be seen in the comparison of the left portion with the rightportion of FIG. 8, such a fluororesin membrane is disadvantageous inthat swelling easily causes methanol crossover and change of theprotonically-conductive structure in the membrane. Thus, methanol cannotbe made sufficient use of, making it impossible to cause stableelectrode reaction. As a result, the electricity generating efficiencyis not sufficient.

It is also disadvantageous in that the repetition of swelling can easilycause deterioration of mechanical strength.

(Patent Reference 1) JP-A-7-90111

DISCLOSURE OF THE INVENTION

The invention has been worked out under these circumstances. An aim ofthe invention is to provide a fuel cell having a high mechanicalstrength and a high efficiency which stays stable over an extendedperiod of time.

The invention also provides a fuel cell which can be easily produced.

Thus, the fuel cell of the invention comprises a porouselectrically-conductive material as a substrate, aprotonically-conductive membrane formed on the porouselectrically-conductive material made of a mesoporous thin filmcomprising as a main component a crosslinked structure having ametal-oxygen skeleton having an acid group connected to at least a partthereof and having pores periodically aligned therein and a porouselectrically-conductive material layer formed on theprotonically-conductive membrane.

In this arrangement, the protonically-conductive membrane is formed by acrosslinked structure having a rigid metal-oxygen skeleton. Thus, theskeleton structure itself is rigid. Accordingly, theprotonically-conductive membrane can keep its pore diameter constantwithout swelling. Thus, crossover of methanol can be eliminated. As aresult, a fuel cell having a high reliability can be provided. Theporous electrically-conductive material as a substrate and the porouselectrically-conductive material layer formed on theprotonically-conductive membrane act as electrode. Since these layerscan be integrally formed by a film-forming process rather than a bondingprocess, they can be easily mounted and exhibit an excellent adhesion.

The invention also concerns the aforementioned fuel cell wherein thecrosslinked structure is formed by a silicon-oxygen bond. In thisarrangement, a rigider skeleton structure can be obtained to provide astructure with a higher reliability.

The invention further concerns the aforementioned fuel cell wherein themesoporous thin film has a thickness of 10 μm or less. In thisarrangement, the protonic conduction distance can be reduced, making itpossible to substantially raise the amount of protonic conduction. As aresult, a fuel cell having a high efficiency can be obtained.

The invention further concerns the aforementioned fuel cell wherein theporous electrically-conductive material is a porous silicon layer formedby the anodization of silicon.

In this arrangement, an ordinary silicon process can be used to form theporous electrically-conductive material easily.

The invention further concerns the aforementioned fuel cell wherein anacid group is connected to the interior of the pores.

In this arrangement, the protonic conductivity can be further raised,making it possible to provide a fuel cell having excellent outputproperties.

The method for producing a fuel cell of the invention comprises a stepof forming a substrate at least the surface of which is a porouselectrically-conductive material, a step of forming aprotonically-conductive membrane made of a mesoporous thin filmcomprising as a main component a crosslinked structure having ametal-oxygen skeleton having an acid group connected to at least a partthereof and having pores periodically aligned therein on the porouselectrically-conductive material and a step of forming a porouselectrically-conductive material layer on the protonically-conductivemembrane.

In accordance with this method, an ordinary semiconductor process can beused to form a fuel cell having a high reliability easily at a goodworking efficiency.

The invention further concerns the aforementioned method for producing afuel cell wherein the step of forming the substrate involves a step ofanodizing the surface of a silicon substrate to form a porous siliconlayer thereon.

In this manner, the silicon substrate is rendered permeable to methanolon the porous silicon layer side thereof. Thus, an electrode having agood fuel permeability can be formed by an ordinary silicon process.

The invention further concerns the aforementioned method for producing afuel cell wherein the anodization step is preceded by a step ofselectively etching the fuel cell forming region to a desired thickness.

In this manner, the thickness of the silicon substrate can be reduced toa value at which the silicon substrate can be fairly anodized all overthe layer, making it possible to form a porous layer free of dispersion.At the same time, the anodization step requires a shorter time. Thedesired thickness of the silicon substrate thus etched is determined bythe etching conditions. Thus, an electrode formed by a porous siliconlayer having uniform thickness and properties can be formed at a goodworking efficiency.

The invention concerns the aforementioned method for producing a fuelcell wherein the step of forming a porous silicon layer is followed by astep of etching the silicon substrate on the back side thereof so thatthe porous silicon layer is reached to form a thin film.

In accordance with this method, the anodization step is followed by thereduction of the thickness of the silicon substrate. Thus, the area leftinsufficiently oxidized or the damaged surface may be merely etchedaway, making it possible to form an electrode formed by a porous siliconlayer having a high reliability at a good working efficiency.

The invention further concerns the aforementioned method for producing afuel cell wherein there comprises a step of introducing an acid group inthe pores.

In accordance with this method, a silicon-oxygen structure can beexposed to an oxidizing atmosphere such as sulfonic acid to introduce anacid group in the pores at a good working efficiency.

The invention further concerns the aforementioned method for producing afuel cell wherein there comprises a step of introducing a particulatecatalyst into the surface of the crosslinked structure.

In accordance with this method, a particulate catalyst can be stablysupported on the surface of the crosslinked structure.

The invention further concerns the aforementioned method for producing afuel cell wherein there comprises a step of forming a first catalystlayer on the surface of the porous silicon layer, a step of forming onthe surface of the catalyst layer a protonically-conductive membranemade of a mesoporous thin film comprising a crosslinked structure havingan inorganic skeleton and having pores aligned periodically therein andan acid group connected thereto and a step of forming a second catalystlayer on the protonically-conductive membrane.

In this manner, a fuel cell can be easily produced at a good workingefficiency.

Further, the porous electrically-conductive material can be formed by aporous carbon to have a good electric conductivity as well as a goodadhesion to the oxygen-silicon crosslinked structure.

Moreover, in the invention, the step of forming aprotonically-conductive membrane may involve a step of preparing aprecursor solution containing water, ethanol, hydrochloric acid, asurface active agent and TEOS, a step of spreading the precursorsolution over a substrate, a step of removing the surface active agentto form a crosslinked structure having a silicon-oxygen structure, astep of silylating the crosslinked structure to form a crosslinkedstructure having a mercapto group in the silicon-oxygen structure and astep of oxidizing the mercapto group in the crosslinked structure toform a crosslinked structure having a sulfonic acid group.

In accordance with this method, the composition ratio of the precursorsolution and the silylation and oxidization conditions can be controlledto control the porosity and the formation of the protonically-conductivechannel. In this manner, the methanol and proton permeability of themembrane can be controlled.

Further, the step of removing the surface active agent may be precededby a step of exposing the silicon substrate to MPTMS vapor to silylatethe silicon substrate.

In this manner, an acid group can be introduced into micropores as well,making it possible to form a protonically-conductive membrane having ahigh protonic conductivity.

Moreover, the step of removing the surface active agent may involve astep of extracting the surface active agent with an acid.

In this manner, the surface active agent can be extracted withoutpassing through a high temperature step, making it possible to extractthe surface active agent without releasing the acid group introduced atthe silylation step.

The invention further concerns the aforementioned method for producing afuel cell wherein the step of removing a surface active agent involves acalcining step.

Calcination causes the surface active agent to be fairly removed, makingit possible to form a crosslinked structure containing a silicon-oxygenstructure.

The invention further concerns the aforementioned method for producing afuel cell wherein the silylation step involves a step of exposing thesilicon substrate to a mercaptopropyl trimethoxy silane (MPTMS) vapor.In this manner, a crosslinked structure having a mercapto groupconnected thereto can be easily formed.

The invention further concerns the aforementioned method for producing afuel cell wherein the step of supplying a precursor solution to thesubstrate involves a step of dipping the substrate in the precursorsolution and withdrawing the substrate from the precursor solution at adesired rate.

Preferably, the step of supplying a precursor solution to the substrateinvolves a step of repeatedly and sequentially spreading the precursorsolution over the substrate.

More preferably, the step of supplying a precursor solution to thesubstrate involves a rotary spreading step of dropping the precursorsolution onto the substrate and rotating the substrate.

In accordance with the aforementioned method, the thickness of the layeror the pore diameter can be adjusted to easily adjust the capability ofinhibiting permeation of methanol and the protonic conductivity, makingit possible to form a high quality fuel cell with a good productivity.

Further, in accordance with the method of the invention, the silicaderivative is selected, making it possible to further adjust theporosity.

As mentioned above, in accordance with the invention, MEA of fuel cellhaving a protonically- conductive membrane formed by a crosslinkedstructure having a rigid metal-oxygen skeleton can be integrally formed,making it possible to provide a stable fuel cell having a highmechanical strength and a high efficiency.

Moreover, in accordance with the invention, a fuel cell can be easilyprovided at a good working efficiency by an ordinary semiconductorprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating the configuration of theprotonically-conductive membrane of a fuel cell formed by an embodimentof implementation of the invention;

FIG. 2 is an enlarged view of an essential part of theprotonically-conductive membrane;

FIGS. 3A to 3F are the diagrams illustrating a process for theproduction of a fuel cell comprising a protonically-conductive membraneaccording to Embodiment 1 of implementation of the invention;

FIG. 4 is a flow chart illustrating a process for the formation of theprotonically-conductive membrane according to Embodiment 1 ofimplementation of the invention;

FIG. 5 is a configurational view illustrating the electrophoresis inEmbodiment 1 of implementation of the invention;

FIG. 6 is a flow chart illustrating a process for the formation of aprotonically-conductive membrane according to Embodiment 2 ofimplementation of the invention;

FIGS. 7A to 7G are the diagrams illustrating a process for theproduction of a fuel cell comprising a protonically-conductive membraneaccording to Embodiment 3 of implementation of the invention; and

FIGS. 8A and 8B are the diagrams illustrating swelling of a related artprotonically-conductive membrane.

In these drawings, the reference numeral 1 denotes a perfluoro group,the reference numeral denotes a sulfonic acid group, and the referencenumeral 3 denotes a protonically-conductive channel.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the fuel cell according to the invention will bedescribed in detail in connection with the attached drawings.

Embodiment 1

As diagrammatically shown in FIG. 1, the protonically-conductivemembrane to be incorporated in the fuel cell according to the presentembodiment is made of a mesoporous thin film comprising as a maincomponent a crosslinked structure having a metal-oxygen skeleton havingan acid group connected to at least a part thereof and having poresperiodically aligned therein to form a

FIG. 2 is an enlarged view of an essential part of FIG. 1. A columnarpore which forms a protonically- conductive channel 3 has a sulfonicacid group incorporated therein to have an enhanced protonicconductivity.

A method for forming the electrode-electrolyte assembly (MEA) of thefuel cell will be described hereinafter.

FIGS. 3A to 3G each depicts a procedure of forming MEA.

FIG. 4 is a flow chart illustrating a procedure of forming aprotonically-conductive membrane.

Firstly, as shown in FIG. 3A, an n-type silicon substrate 11 having a(100) plane having a specific resistance of 5×10¹⁸ cm⁻³ as a main planeis prepared.

Subsequently, as shown in FIG. 3B, a resist pattern having an opening ina cell forming region is formed on the back side of the siliconsubstrate 11. The silicon substrate 11 is then subjected to anisotropicetching with a 83° C. TMAH solution to a desired depth. In this manner,an opening 12 is formed for forming a thin portion.

Subsequently, as shown in FIG. FIG. 3C, the silicon substrate isanodized so that the entire silicon substrate 11 becomes a poroussilicon 13 having a pore diameter of from 10 nm to 5 μm.

Subsequently, a mesoporous thin film (protonically-conductive membrane)having columnar pores aligned periodically and perpendicular to thesurface of the silicon substrate is formed on the porous silica 13.

In some detail, a cationici cetyl trimethyl ammonium bromide (C16TAB:C₁₆H₃₃N⁺ (CH₃)₃Br) as a surface active agent, TEOS (tetraethoxysilane)as a silica derivative and hydrochloric acid (HCl) as an acid catalystare dissolved in a mixture of H₂O (water) and Et-OH (alcohol), and thenstirred in a mixing vessel to prepare a precursor solution. The molarratio of these components in the precursor solution (H₂OEt-OH:HCl:C16TAB:TEOS) is 100:76:5:0.5:3. The mixed solution thusprepared is spread over the surface of the silicon substrate on whichthe porous silicon 13 has been formed using a spinner as shown in FIG.3B (Step 101 in FIG. 4), and then dried at 90° C. for 5 minutes (Step102 in FIG. 4) so that the silica derivative is subjected to hydrolyticpolycondensation reaction to undergo polymerization (precrosslinkingstep). Thus, a periodic self-agglomerate of surface active agent isformed.

This self-agglomerate forms a rod-shaped micelle structure having aplurality of molecules C₁₆H₃₃N⁺ (CH₃)₃Br agglomerated. In thisarrangement, as the percent agglomeration rises with the rise ofconcentration, the portion freed of methyl group becomes more hollow.Thus, a crosslinked structure having pores aligned therein is formed.

Subsequently, the self-agglomerate is washed with water, dried, and thenheated/calcined in a 500° C. nitrogen atmosphere for 6hours (Step 103 inFIG. 4) so that the surface active agent in the matrix is fullythermally decomposed away to form a pure mesoporous thin film. Thesilicon substrate is then processed with a 180° C. MPTMS vapor for 4hours (Step 104 in FIG. 4) to form a silicon- oxygen crosslinkedstructure having mercapto group connected thereto. Thereafter, thesilicon substrate is subjected to heat treatment in a 30% hydrogenperoxide for 30 minutes (Step 105 in FIG. 4), and then dried (Step 106in FIG. 4).

In this manner, a protonically-conductive membrane 14 is formed as shownin FIG. 3D. The protonically-conductive membrane 14 has columnar poresaligned along the thickness of the layer.

FIG. 2 is a configurational view illustrating a section of thisstructure. As can be seen in FIG. 2, columnar pores are formed. Further,a porous thin film having a skeleton structure containing a large numberof pores is formed.

Thereafter, a carbon-supported platinum, a 5 wt-% Nafion® solution andethanol are mixed, and then subjected to supersonic dispersion toprepare a suspension A. As shown in FIG. 5, a voltage is then applied tothe system with the suspension A in contact with the back side of theporous silicon 13 and a 0.1 M aqueous solution of perchloric acid Bprovided on the other side to cause electrophoresis by which a catalystlayer 15 is formed. During this process, Nafion® in the suspension isattached to the surface of the porous silicon 13 to act as a dispersant.Thus, the catalyst layer 15 containing platinum is formed.

Subsequently, as shown in FIG. 3E, a catalyst layer 16 is similarlyformed on the front surface of the protonically-conductive membrane 14.

Subsequently, as shown in FIG. 3F, an electrode layer 17 is formed.

In this manner, MEA is formed. A diffusion electrode (not shown) is thenattached to this MEA to form a DMFC type fuel cell.

In this arrangement, the protonically-conductive membrane is formed by asilicon-oxygen crosslinked structure having columnar pores alignedregularly and thus has a high mechanical strength and undergoes noswelling. Further, since this protonically-conductive membrane undergoesno swelling, it undergoes little methanol crossover and exhibits a highefficiency and a high reliability.

By subjecting the silicon substrate to processing with TEOS vapor priorto calcination, the volumetric shrinkage during calcination can beeliminated to strengthen the silica skeleton, making it possible tofurther enhance the mechanical strength thereof.

While the present embodiment has been described with reference to thecase where the protonically-conductive membrane is formed by aninorganic structure mainly composed of a crosslinked structurecontaining a silicon-oxygen bond, an organic- inorganic hybridcrosslinked structure containing an organic group in the silicon-oxygenskeleton may be used.

By covering the back side of MEA with a glass substrate or the like toform a fuel channel and forming a metal electrode on the surface of theelectrode layer 17 on the front surface thereof, a fuel cell module canbe formed.

While the present embodiment has been described with reference to thecase where only MEA is formed by a silicon process, a continuous siliconprocess may be effected on a wafer level to form other portions,including a groove for fuel channel. The back side of the structureshown in FIG. 3F may be covered with a glass substrate or the like toform a fuel channel. A metal electrode may be then formed on the surfaceof the electrode layer 17 on the front surface of the structure. Thestructure may be then diced to form individual cells.

In this manner, a fuel cell module can be easily formed.

Embodiment 2

In Embodiment 1, silylation is effected after calcination. In thepresent embodiment, however, silylation is effected prior to extractionof surface active agent by calcination as shown in the flow chart ofFIG. 6 to introduce an acid group (mercapto group) also into thesilicon-oxygen skeleton so that the surface active agent can besubsequently extracted with hydrochloric acid.

As shown in the flow chart of FIG. 6, a cationici cetyl trimethylammonium bromide (C16TAB: C₁₆H₃₃N⁺ (CH₃)₃Br) as a surface active agent,TEOS (tetraethoxysilane) as a silica derivative and hydrochloric acid(HCl) as an acid catalyst are dissolved in a mixture of H₂O (water) andEt-OH (alcohol), and then stirred in a mixing vessel to prepare aprecursor solution. The molar ratio of these components in the precursorsolution (H₂O:Et-OH:HCl:C16TAB:TEOS) is 100:76:5:0.5:3. The mixedsolution thus prepared is spread over the surface of the siliconsubstrate on which the porous silicon 13 has been formed using a spinneras shown in FIG. 3B (Step 201 in FIG. 6), and then dried at 90° C. for 5minutes (Step 202 in FIG. 6) so that the silica derivative is subjectedto hydrolytic polycondensation reaction to undergo polymerization(precrosslinking step). Thus, a periodic self-agglomerate of surfaceactive agent is formed.

This self-agglomerate forms a rod-shaped micelle structure having aplurality of molecules C₁₆H₃₃N⁺ (CH₃)₃Br agglomerated. In thisarrangement, as the percent agglomeration rises with the rise ofconcentration, the portion freed of methyl group becomes more hollow.Thus, a crosslinked structure having pores aligned therein is formed.

Subsequently, the self-agglomerate is exposed to MPTMS vapor so that anacid group is introduced also into the silicon-oxygen skeleton (Step 203in FIG. 6), washed with water, dried, and then extracted withhydrochloric acid (Step 204 in FIG. 6) so that the surface active agentin the matrix is fully decomposed away to form a pure mesoporous thinfilm.

The silicon substrate is then again processed with a 180° C. MPTMS vaporfor 4 hours (Step 205 in FIG. 6) to form a silicon-oxygen crosslinkedstructure having mercapto group connected thereto. Thereafter, thesilicon substrate is subjected to heat treatment in a 30% hydrogenperoxide for 30 minutes (Step 206 in FIG. 6), and then dried (Step 207in FIG. 6).

In this manner, in addition to the advantage of Embodiment 1, acidgroups are introduced prior to the removal of the surface active agent,making it possible to incorporate more acid groups in the structure.Thus, a protonically-conductive membrane having a high reactivity can beobtained.

The formulation of the precursor solution is not limited to that of thepresent embodiment. The composition ratio of the surface active agent,the silica derivative and the acid catalyst are preferably from 0.01 to0.1, from 0.01 to 0.5 and from 0 to 5 based on 100 of the solvent. Theuse of the precursor solution having such a formulation makes itpossible to form a membrane having cylindrical pores.

While the present embodiment has been described with reference to thecase where as the surface active agent there is used a cationic cetyltrimethyl ammonium bromide (CTAB: C₁₆H₃₃N⁺ (CH₃)₃Br), the invention isnot limited thereto. It goes without saying that other surface activeagents such as nonionic Pluronic HO—CH₂CH(CH₃)O)_(y)—CH₂CH₂)O)_(x)—H maybe used.

However, when an alkali ion such as Na ion is used as a catalyst, itcauses deterioration of semiconductor material. Therefore, a cationicsurface active agent is preferably used. As a catalyst there ispreferably used an acid catalyst. As such an acid catalyst there may beused nitric acid (HNO₃), sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄),H₄SO₄ or the like besides HCl.

The silica derivative is not limited to hydrogen silosesquioxane (HSQ)or methyl silosesquioxane (MSQ). Any silica derivative materials havinga 4-membered or higher siloxane skeleton may be used.

While the present embodiment has been described with reference to thecase where as a solvent there is used a mixture of water (H₂O) andalcohol, only water may be used.

While the present embodiment has been described with reference to thecase where as a calcining atmosphere there is used a nitrogenatmosphere, calcination may be effected in vacuo or in the atmosphere.Preferably, the use of a forming gas composed of a mixture of nitrogenand hydrogen makes it possible to enhance the moisture resistance andhence reduce leakage current.

The mixing proportion of surface active agent, silica derivative, acidcatalyst and solvent can be properly changed.

While the present embodiment has been described with reference to thecase where the prepolymerization step is carried out by keeping thereaction mixture at a temperature of from 30° C. to 150° C. for 1 hourto 120 hours, the reaction temperature is preferably from 60° C. to 120°C., more preferably 90° C.

While the present embodiment has been described with reference to thecase where the calcination step is effected at 500° C. for 6 hours, thecalcination step may be effected at a temperature of from 250° C. to500° C. for 1 to 8 hours, preferably from 350° C. to 450° C. for about 6hours.

Even when the same processing is effected, different results areobtained from the case where a surface active agent is used to the casewhere no surface active agent is used. In other words, at the step ofMPTMS processing (Step 203) effected prior to the removal of surfaceactive agent, the silylating agent penetrates into and modifies thesilica because the micropores have the surface active agent presenttherein. On the other hand, at the step of MPTMS processing (Step 205)effected after the removal of surface active agent, the silylating agentdiffuses the pores and modifies the surface of the micropores.

Embodiment 3

While Embodiment 1 has been described with reference to the case wherethe formation of the catalyst layer is carried out by electrophoresis,the formation of the catalyst layer may be carried out by plating in thepresent embodiment as shown in the flow sheet of FIGS. 7A to 7G.

As shown in FIGS. 7A to 7C, the processing is effected in the samemanner as in Embodiment 1 until the step of reducing the thickness ofthe silicon substrate 11 to form a porous silicon 13.

Subsequently, as shown in FIG. 7D, plating is effected to form acatalyst layer 25 made of a metal containing platinum on the poroussilicon 13.

Subsequently, as shown in FIG. 7E, a mesoporous thin film(protonically-conductive membrane) 24 having columnar pores alignedperiodically and perpendicular to the surface of the silicon substrateis formed in the same manner as in Embodiment 1.

Subsequently, as shown in FIG. 7F, plating is effected to form acatalyst layer 26 made of a metal containing platinum on theprotonically-conductive membrane 24.

As shown in FIG. 7G, a paste containing a particulate carbon is spreadover the catalyst layer, and then calcined to form an electrode layer27.

Thus, MEA is formed.

Embodiment 4

While Embodiment 1 has been described with reference to the case wherethe formation of the mesoporous thin film is carried out by a spincoating method, the invention is not limited thereto. A dipping methodmay be used.

In some detail, a cationici cetyl trimethyl ammonium bromide (CTAB:C₁₆H₃₃N⁺ (CH₃)₃Br) as a surface active agent, hydrogen silosesquioxane(HSQ) as a silica derivative and hydrochloric acid (HCl) as an acidcatalyst are dissolved in a mixture of H₂O and alcohol, and then stirredin a mixing vessel to prepare a precursor solution. The molar ratio ofthese components in the precursor solution (surface active agent:silicaderivative:acid catalyst) is 0.5:0.01:2 based on 100 of the solvent. Thesilicon substrate 11 on which the porous silicon layer 13 has beenformed is dipped in the mixed solution. The mixing vessel is thensealed. The silicon substrate 11 is then kept at a temperature of from30° C. to 150° C. for 1 hour to 120 hours so that the silica derivativeis subjected to hydrolytic polycondensation reaction to undergopolymerization (precrosslinking step). Thus, a periodic self-agglomerateof surface active agent is formed.

This self-agglomerate forms a rod-shaped micelle structure having aplurality of molecules C₁₆H₃₃N⁺ (CH₃)₃Br agglomerated. In thisarrangement, as the percent agglomeration rises with the rise ofconcentration, the portion freed of methyl group becomes more hollow.Thus, acrosslinked structure having pores aligned therein is formed.

Subsequently, the silicon substrate is withdrawn from the mixedsolution, washed with water, dried, and then heated/calcined in a 400°C. nitrogen atmosphere for 3 hours so that the surface active agent inthe matrix is fully thermally decomposed away to form a pure mesoporousthin film.

Embodiment 5

While Embodiment 1 has been described with reference to the case wherethe formation of the mesoporous thin film is carried out by a spincoating method, the invention is not limited thereto. A dip coatingmethod may be used.

In some detail, the silicon substrate is allowed to descendperpendicular to the liquid level of the precursor solution prepared ata rate of from 1 mm/s to 10 m/s until it sinks in the solution, and thenallowed to stand for 1 second to 1 hour.

After the lapse of a desired period of time, the silicon substrate isthen allowed to ascend vertically at a rate of from 1 mm/s to 10 m/suntil it is withdrawn from the solution.

Finally, the silicon substrate is calcined in the same manner as inEmbodiment 1 so that the surface active agent in the matrix is fullythermally decomposed away to form a pure mesoporous thin film.

While the present embodiment has been described with reference to thecase where a mesoporous thin film having columnar pores periodicallyaligned is used, the diameter and alignment of the pores are not limitedto the present embodiment but may be changed.

As the catalyst there may be used Brij30 (C₁₂H₂₅ (OCH₂CH₂)₄OH) or thelike besides C16TAB.

Further, the use of Pluronic F127 (trade name) as a surface active agentmakes it possible to form a three-dimensional porous thin film.

While the present embodiment has been described with reference to acrosslinked structure having a silicon-oxygen bond, a metal-oxygencrosslinked structure such as titanium-oxygen crosslinked structure maybe also used.

Moreover, as the acid group which is bonded to the silicon-oxygencrosslinked structure to cause protonic conduction there may be usedphosphoric acid (H₃PO₄) or perchloric acid (HClO₄) besides sulfonicacid.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

The present application is based on Japanese Patent Application No.2003-320968, filed on Sep. 12, 2003, the contents of which are herebyincorporated by reference.

INDUSTRIAL APPLICABILITY

As mentioned above, the invention can be effectively applied to DMFCtype fuel cell and thus can be effectively used as an electric supplyfor small-sized apparatus such as cellular phone and note personalcomputer.

1. A fuel cell comprising a porous electrically-conductive material as asubstrate, a protonically-conductive membrane formed on the porouselectrically-conductive material made of a mesoporous thin filmcomprising as a main component a crosslinked structure having ametal-oxygen skeleton having an acid group connected to at least a partthereof and having pores periodically aligned therein and a porouselectrically-conductive material layer formed on theprotonically-conductive membrane.
 2. The fuel cell as defined in claim1, wherein the crosslinked structure is formed by a silicon-oxygen bond.3. The fuel cell as defined in claim 1, wherein the mesoporous thin filmhas a thickness of 10 μm or less.
 4. The fuel cell as defined in anyclaim 1, wherein the porous electrically-conductive material is a poroussilicon layer formed by the anodization of silicon.
 5. A method forproducing a fuel cell comprising: A step of forming a substrate at leastthe surface of which is a porous electrically-conductive material; Astep of forming a protonically-conductive membrane made of a mesoporousthin film comprising as a main component a crosslinked structure havinga metal-oxygen skeleton having an acid group connected to at least apart thereof and having pores periodically aligned therein on the porouselectrically-conductive material; and A step of forming a porouselectrically-conductive material layer on the protonically-conductivemembrane.
 6. The method for producing a fuel cell as defined in claim 5,wherein the step of forming the substrate involves a step of anodizingthe surface of a silicon substrate to form a porous silicon layerthereon.
 7. The method for producing a fuel cell as defined in claim 6,wherein the anodization step is preceded by a step of selectivelyetching the fuel cell forming region to a desired thickness.
 8. Themethod for producing a fuel cell as defined in claim 6, wherein the stepof forming a porous silicon layer is followed by a step of etching thesilicon substrate on the back side thereof so that the porous siliconlayer is reached to form a thin film.